Abstract

Seismic lithology and particularly amplitude changes with offset (AVO) has received worldwide attention during the last few years. This paper shows the techniques behind two new computer programs PLANET and EQUATER, developed at PT Caltex Pacific Indonesia (CPI).

The first program, Planet, estimates shear wave velocities, models the effect of varying fluid content in the rock on its' velocity and density and graphs the changes in reflection coefficient as a function of angle of incidence.

EQUATER (EQUal Angle Trace Envelope Relation) is a data processing program which performs weighted stacks equivalent to making least square fits to time slic amplitudes of the NMO corrected CMP gathers for every time slice. With the use of ray tracing and an input velocity model, these weighted stacks simulate data gathered at a constant angle of incidence.

Typically, two stacks are compared for a small angle (5°) and a larger angle (30°). The differnce in amplitudes of the two stacks can be compared to the theoretical result from the PLANET program. Two examples of the process are shown over known gas accumulations. The first is from the Sikladi area with gas in the shallow Petani A and B sands. In this case the amplitude of these events increases with offset when proper amplitude and array attenuation corrections are applied. The second example, from the Jambi Basin over a gas well, shows a similar strong amplitude increase with offset.